During venipuncture, the butterfly is held by its wings between thumb and index finger. This grasp very close to the needle facilitates precise placement. The needle is generally inserted toward the vein at a shallow angle, made possible by the set's design. When the needle enters the vein, venous blood pressure generally forces a small amount of blood into the set's transparent tubing providing a visual sign, called the "flash" or "flashback", that lets the practitioner know that the needle is actually inside of a vein.